If it has to be LAN only, you will have a problem with it. Currently, this cannot be done without the Internet.
Theoretically, it could be possible to do this locally, but it is a complete step beyond the ewelink ecosystem of scenes.
The Sonoff device itself probably won’t be able to handle it. You need to have some central point, such as a server, that will run the logic flow…
I doubt whether devs will implement such functionality.